Engine torque smoothing

ABSTRACT

Methods, devices, estimators, controllers and algorithms are described for estimating the torque profile of an engine and/or for controlling torque applied to a powertrain by one or more devices other than the engine itself to manage the net torque applied by the engine and other device(s) in manners that reduce undesirable NVH. The described approaches are particularly well suitable for use in hybrid vehicles in which the engine is operated in a skip fire or other dynamic firing level modulation manner—however they may be used in a variety of other circumstances as well. In some embodiments, the hybrid vehicle includes a motor/generator that applies the smoothing torque.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.17/002,309 filed Aug. 25, 2020, which is a Continuation of U.S.application Ser. No. 16/278,075 filed Feb. 16, 2019 (now U.S. Pat. No.10,787,979 issued Sep. 29, 2020), which is a Continuation of U.S.application Ser. No. 16/038,622, filed on Jul. 18, 2018 (now U.S. Pat.No. 10,436,133, issued Oct. 8, 2019), which is a Continuation of U.S.application Ser. No. 15/679,462, filed on Aug. 17, 2017 (now U.S. Pat.No. 10,060,368, issued on Aug. 28, 2018), which claims priority of U.S.Provisional Patent Application No. 62/379,357, filed Aug. 25, 2016. U.S.application Ser. No. 15/679,462 is also a Continuation-in-Part of U.S.application Ser. No. 15/340,291, filed on Nov. 1, 2016 (now U.S. Pat.No. 10,221,786, issued Mar. 5, 2019), which is a Continuation of U.S.application Ser. No. 14/992,779, filed on Jan. 11, 2016 (now U.S. Pat.No. 9,512,794, issued on Dec. 6, 2016). U.S. application Ser. No.14/992,779 claims priority of U.S. Provisional Patent Application Nos.62/102,206, filed on Jan. 12, 2015, and 62/137,539, filed on Mar. 24,2015. Each of these referenced priority applications are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to hybrid vehicles powered byinternal combustion engines operating under skip-fire control havinganother source of power in addition to the internal combustion engine.The torque profile of the skip-fire controlled engine is estimated andthe additional source of power is used to smooth the torque profile.

BACKGROUND

Fuel efficiency of internal combustion engines can be substantiallyimproved by varying the displacement of the engine. This allows for thefull torque to be available when required, yet can significantly reducepumping losses and improve thermal efficiency by using a smallerdisplacement when full torque is not required. The most common methodtoday of implementing a variable displacement engine is to deactivate agroup of cylinders substantially simultaneously. In this approach theintake and exhaust valves associated with the deactivated cylinders arekept closed and no fuel is injected when it is desired to skip acombustion event. For example, an 8 cylinder variable displacementengine may deactivate half of the cylinders (i.e. 4 cylinders) so thatit is operating using only the remaining 4 cylinders. Commerciallyavailable variable displacement engines available today typicallysupport only two or at most three displacements.

Another engine control approach that varies the effective displacementof an engine is referred to as “skip-fire” engine control. In general,skip-fire engine control contemplates selectively skipping the firing ofcertain cylinders during selected firing opportunities. Thus, aparticular cylinder may be fired during one engine cycle and then may beskipped during the next engine cycle and then selectively skipped orfired during the next. In this manner, even finer control of theeffective engine displacement is possible. For example, firing everythird cylinder in a 4 cylinder engine would provide an effectivedisplacement of ⅓^(rd) of the full engine displacement, which is afractional displacement that is not obtainable by simply deactivating aset of cylinders.

U.S. Pat. No. 8,131,445 (which is incorporated herein by reference)teaches a skip-fire operational approach, which allows any fraction ofthe cylinders to be fired on average using individual cylinderdeactivation. In other skip-fire approaches a particular firing sequenceor firing density may be selected from a set of available firingsequences or fractions. In a skip-fire operational mode the amount oftorque delivered generally depends heavily on the firing density, orfraction of combustion events that are not skipped. Dynamic skip fire(DSF) control refers to skip-fire operation where the fire/skipdecisions are made in a dynamic manner, for example, at every firingopportunity, every engine cycle, or at some other interval.

In some applications referred to as multi-level skip fire, individualworking cycles that are fired may be purposely operated at differentcylinder outputs levels—that is, using purposefully different air chargeand corresponding fueling levels. By way of example, U.S. Pat. No.9,399,964 (which is incorporated herein by reference) describes somesuch approaches. The individual cylinder control concepts used indynamic skip fire can also be applied to dynamic multi-charge levelengine operation in which all cylinders are fired, but individualworking cycles are purposely operated at different cylinder outputlevels. Dynamic skip fire and dynamic multi-charge level engineoperation may collectively be considered different types of dynamicfiring level modulation engine operation in which the output of eachworking cycle (e.g., skip/fire, high/low, skip/high/low, etc.) isdynamically determined during operation of the engine, typically on anindividual cylinder working cycle by working cycle (firing opportunityby firing opportunity) basis. It should be appreciated that dynamicfiring level engine operation is different than conventional variabledisplacement in which when the engine enters a reduced displacementoperational state, a defined set of cylinders are operated in generallythe same manner until the engine transitions to a different operationalstate.

The combustion process and the firing of cylinders using skip fire orother firing level modulation techniques can introduce unwanted noise,vibration and harshness (NVH). For example, the engine can transfervibration to the body of the vehicle, where it may be perceived byvehicle occupants. Sounds may also be transmitted through the chassisinto the vehicle cabin. Under certain operating conditions, the firingof cylinders generates undesirable acoustic effects through the exhaustsystem and tailpipe. Vehicle occupants may thus experience undesirableNVH from structurally transmitted vibrations or air transmitted sounds.

A challenge with skip fire engine control is obtaining acceptable NVHperformance. While prior approaches work well, there are continuingefforts to develop new and improved approaches for managing NVH duringfiring level modulation operation of an engine.

SUMMARY

A variety of methods, devices, estimators, controllers and algorithmsare described for estimating the torque profile of an engine and/or forcontrolling torque applied to a powertrain by one or more devices otherthan the engine itself to manage the net torque applied by the engineand other device(s) in manners that reduce undesirable NVH. Thedescribed approaches are particularly well suitable for use in hybridvehicles in which the engine is operated in a skip fire or other dynamicfiring level modulation manner—however they may be used in a variety ofother circumstances as well. In some embodiments, the hybrid vehicleincludes a motor/generator that applies the smoothing torque.

In some embodiments, periods are identified in which an instantaneoustorque or an instantaneous acceleration produced by the engine isexpected to exceed a designated threshold. A counteracting torque isthen applied to the powertrain in a controlled manner by an energysource or sink during the identified periods such that the expected netpowertrain torque does not exceed the designated threshold. In someembodiments, the designated threshold may vary as a function of enginespeed and/or transmission gear. In some embodiments, the counteracting(smoothing) torque is applied in short impulses timed to counteracttorque spikes generated during skip fire or dynamic firing levelmodulation operation of the engine.

In some hybrid vehicle embodiments, when an estimated engine torqueprofile is determined to provide acceptable NVH, the hybrid vehicle isoperated solely on the output of the internal combustion engine.However, when the estimated engine torque profile is determined toprovide unacceptable NVH, the both the internal combustion engine and anauxiliary power source/sink are utilized, with the auxiliary powersource/sink being arranged to provide a smoothing torque to reduce NVHto an acceptable level.

In some embodiments, the overall engine torque profile and thedetermination of the counteracting smoothing torque is updated eachfiring opportunity such that need for and magnitude of the counteractingsmoothing torque is updated for each firing opportunity.

In some skip fire or other dynamic firing level modulation embodiments,the torque profile estimations are used in the selection of the(effective) operational firing fraction. In such embodiments, the fuelefficiency of various candidate firing fractions may be compared afterconsidering the fuel efficiency implications of any smoothing torquesthat may be required when operating at the respective firing fractionsto meet desired drivability criteria.

In some embodiments, the torque profile for the engine may be determinedby summing the contribution of each of the working chambers (e.g.cylinders). In some embodiments, the torque profile for a particularcylinder may be accomplished by selecting or determining a normalizedtorque profile for the cylinder's operational state and then scaling thenormalized torque profile based on current engine operating parameters.During skip fire engine operation, the normalized torque profileutilized will vary based on the skip/fire firing decision for thatparticular cylinder. In some embodiments, the normalized torque profilewill be based at least in part on intake manifold pressure. In someembodiments, the normalized torque profile may be scaled based on one ormore current operating parameters such as engine speed, sparking timing,valve timing/lift, engine firing history, cylinder firing history, etc.

In some embodiments, the engine torque profile is filtered to identifyselected harmonic components of the torque profile. A counteractingsmoothing torque to apply to the powertrain may then be based on thefiltered results. In some such embodiments, the filtered results may beamplified based on one or more current engine parameters. The filteredsignal may be delayed to align with the torque predicted to be producedby the engine. The amplified filtered signal may be inverted and used inthe control of an electric motor/generator to source/sink torque basedon the inverted torque signal.

In some embodiments the smoothing torque may be applied as one or moreoscillating (e.g. sinusoidal) signal, whereas in others the smoothingtorque may be applied as impulses intended to offset portions ofexpected torque spikes.

In various embodiments, the smoothing torque can effectively be appliedby devices that draw energy from the powertrain by increasing ordecreasing their respective loads appropriately. Similarly the torqueapplied by devices that add torque to the powertrain can be increased ordecreased to effectively provide the desired smoothing torque. Whendevices such as a motor/generator that can both add and subtract torqueare used, either of these approaches may be used or the devices may bevaried between torque contributing and torque drawing states to providethe desired smoothing torque.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a diagrammatic illustration of a representative hybridpowertrain according to an embodiment of the present invention.

FIG. 2 is a diagrammatic illustration of a representative controlarchitecture for a hybrid powertrain according to an embodiment of thepresent invention.

FIGS. 3A and 3B show a cylinder torque profile versus crank angle formultiple firings at different MAP values.

FIGS. 4A and 4B show a normalized torque profile versus crank angle fora combustion stroke at different MAP values according to an embodimentof the present invention.

FIGS. 5A and 5B show a normalized torque profile versus crank angle fora compression stroke at different MAP values according to an embodimentof the present invention.

FIG. 6 shows an exemplary table showing values for the normalized torqueprofile for different values of MAP according to an embodiment of thepresent invention.

FIG. 7 shows an exemplary table showing the torque scaling factor fordifferent values of MAP and engine speed according to an embodiment ofthe present invention.

FIG. 8 shows an exemplary torque profile versus crank angle at anaverage engine speed of 1500 rpm and firing fraction of ¾ for a 4cylinder engine according to an embodiment of the present invention.

FIG. 9 shows the torque profile of FIG. 8 converted into the time domainaccording to an embodiment of the present invention.

FIG. 10 shows the amount of the torque being added to the powertrain(positive value) and removed from the powertrain (negative value) by thesecond power source/sink of the hybrid engine according to an embodimentof the present invention.

FIG. 11 shows a comparison of total power train torque between internalcombustion engine only operation and operation of the engine inconjunction with a second power source according to an embodiment of thepresent invention.

FIG. 12 is an exemplary schematic flow diagram of a method to select themost fuel efficient firing sequence according to an embodiment of thepresent invention.

FIG. 13 is an exemplary schematic flow diagram of a harmoniccancellation method according to an embodiment of the present invention.

FIG. 14 shows a timeline illustrating the timing of a smoothing torquedetermination for a particular working cycle relative to the associatedworking cycle according to an embodiment of the present invention.

FIG. 15 shows exemplary filter characteristics according to anembodiment of the present invention.

FIG. 16 shows a representative engine torque profile and resultantfiltered signal appropriate for driving an additional power source/sinkaccording to an embodiment of the present invention.

FIG. 17 shows suppression of the first and second order frequenciesaccording to an embodiment of the present invention.

FIGS. 18 A-D show an example of cross fading during a firing fractiontransition according to an embodiment of the present invention.

In the drawings, like reference numerals are sometimes used to designatelike structural elements. It should also be appreciated that thedepictions in the figures are diagrammatic and not to scale.

DETAILED DESCRIPTION

The present invention describes methods and systems for reducing NVH andimproving fuel efficiency in a hybrid engine using a skip fire or firinglevel modulation controlled internal combustion engine as one source ofpower. An auxiliary power source/sink is capable to adding and/orremoving torque from the powertrain in a controlled manner that helpsreduce engine generated NVH.

Skip fire operation most commonly includes cylinder deactivation wherebyintake and exhaust valves are kept closed during the nominal gasexchange phases of a 4-stroke engine cycle. Performing cylinderdeactivation requires the engine controller to control power driveroutputs that actuate the cylinder deactivation elements. For camoperated valves, cylinder deactivation may be realized by actuatingsolenoids that operate hydraulic oil control valves, which allow thevalve lifters to either remain rigid (a fired cylinder) or collapse (askipped cylinder). Such a system may be referred to as a “lost-motion”deactivation system. Cylinder deactivation can be achieved using othermechanisms for cam operated valves. Alternatively electromechanicalactuators may be used to control the intake and/or the exhaust valves.Independent of the cylinder deactivation method there is a time lagbetween making a fire/no-fire decision and intake valve opening of afiring cylinder.

The varying and sometimes irregular firing pattern in a skip firecontrolled internal combustion engine can lead to unacceptable NVH withsome firing patterns. One approach to dealing with such problems is tonot use particular firing fractions or firing sequences that are knownto produce unacceptable NVH levels. Instead, other firing fractions orfiring sequences are used and the cylinder output is adjustedaccordingly (e.g., by adjusting the manifold absolute pressure, sparkadvance, etc.) so that the desired engine output is delivered. Theseallowed firing fractions are chosen based on their desirable NVHproperties, i.e. the NVH produced while operating at these firingfractions is acceptable. Various approaches of this kind are describedin co-assigned U.S. patent application Ser. Nos. 13/654,244 and14/638,908, which are incorporated herein in their entirety for allpurposes. Co-assigned U.S. patent application Ser. No. 14/992,779, whichis incorporated herein in its entirety for all purposes, describes somesystems and methods for integrating an additional power source/sink witha dynamic skip fired controlled engine. Forcing a skip fire engine tooperate at only a limited number of firing fractions reduces the fuelefficiency gains that can be realized with skip fire control, sincetorque control must use other actuators such as spark timing, MAP, andcam. Use of these other actuators to control torque output is generallyless fuel efficient than control based exclusively on the firingfraction.

This application describes various control methods in which a secondpower source/sink, in addition to the internal combustion engine, isoperated in a manner that generate a smoothing torque that is applied toa vehicle powertrain. The smoothing torque is any torque that is appliedto help cancel out or reduce a variation in torque generated by theinternal combustion engine. The smoothing torque can be generated by anysuitable energy storage/capture/release device. One example would be anelectric motor/generator with a battery and/or capacitor to store andrelease energy. Alternatively any system or device that stores andcaptures/releases energy mechanically, pneumatically or hydraulicallymay be used. For example, a flywheel with a variable mechanicalcoupling, or a high pressure fluid reservoir with valves controllingfluid flow to and from a turbine or similar device may be used tocapture/release energy from a powertrain. The smoothing torque isapplied in a manner such that noise and vibration generated by the skipfire firing sequence is at least partially reduced or canceled out.

FIG. 1 schematically illustrates an exemplary hybrid electric vehiclepowertrain and associated components that can be used to in conjunctionwith the present invention. These figures shows a parallel hybridelectric powertrain configuration however, it should be appreciated thatthe same concepts can be applied to other hybrid powertrains includingseries hybrid electric configurations, power-split electricconfigurations and hydraulic hybrid configurations, although the largestimprovements in fuel efficiency are expected for the parallel and serieselectric hybrid configurations.

FIG. 1 show a skip fire controlled engine 10 applying torque to apowertrain drive shaft which is connected to a transmission 12, which inturn drives selected wheels 20 of a vehicle. A motor/generator 14 isalso coupled to the powertrain and is capable of either simultaneouslygenerating electrical power (thereby effectively subtracting torque fromthe drive shaft) or supplementing the engine torque, depending onwhether the engine is producing surplus torque or deficit torquerelative to a desired powertrain torque output. When the engine producessurplus torque, the surplus torque causes the motor/generator 14 togenerate electricity which gets stored in the energy storage device 24,which may be a battery and/or a capacitor, after conditioning by thepower electronics 26. The power electronics 26 may include circuitry toconvert the output voltage on the energy storage device 24 to a voltagesuitable for delivering/receiving power from the motor/generator 14.When the engine produces deficit torque the engine torque issupplemented with torque produced by the motor/generator 14 using energypreviously stored in the energy storage device 24. Use of a capacitor asenergy storage device 24 may lead to a larger improvement of the overallfuel economy of the vehicle, since it largely avoids the energy lossesassociated with charging and discharging conventional batteries, whichis particularly advantageous when relatively frequent storage andretrieval cycles are contemplated as in the current invention.

FIG. 2 shows a hybrid vehicle control system suitable for controllingthe hybrid vehicle powertrain shown in FIG. 1 according to a particularembodiment. The vehicle control system 100 includes an engine controlunit (ECU) 130, an internal combustion engine 150, a powertrain 142, andan additional power source/sink 140. The additional power source/sinkmay include power electronics, a motor/generator, and an energy storagedevice. The ECU 130 receives an input signal 114 representative of thedesired engine output. The input signal 114 may be treated as a requestfor a desired engine output or torque. The signal 114 may be received orderived from an accelerator pedal position sensor (APP) 163 or othersuitable sources, such as a cruise controller, a torque calculator, etc.An optional preprocessor 105 may modify the accelerator pedal signalprior to delivery to the engine controller 130. However, it should beappreciated that in other implementations, the accelerator pedalposition sensor may communicate directly with the engine controller 130.

The ECU 130 may include a firing sequence generator 202, a torque modelmodule 204, a power train parameter module 206, a firing control unit210, and an NVH reduction module 208. These units and modulescommunicate with each other and work cooperatively to control thevehicle. The firing sequence generator 202 determines the sequence ofskips and fires of the cylinders of engine 150. The firing sequence maybe generated based on a firing fraction and an output of a delta-sigmaconverter or may be generated in any appropriate manner such asdescribed in U.S. Pat. Nos. 8,099,224, 9,086,020, and 9,200,587, whichare incorporated herein by reference in their entirety. In operation thefiring sequence generator may investigate the fuel efficiency associatedwith various firing sequences and chose the firing sequence that offersoptimal fuel economy while meeting the torque request. In some cases thepowertrain torque may be supplemented or reduced by the powersource/sink 140. The output of the firing sequence generator is a drivepulse signal 113 that may consist of a bit stream, in which each 0indicates a skip and each 1 indicates a fire for an associated cylinderfiring opportunity thereby defining a firing sequence. The firingdecision associated with any firing opportunity is generated in advanceof the firing opportunity to provide adequate time for the firingcontrol unit 210 to correctly configure the engine 150, for example,deactivate a cylinder intake valve on a skipped firing opportunity. Thetorque model module 204 determines an estimated torque based on thefiring sequence and power train parameters determined by the powertrainparameter module 206. These power train parameters may include, but arenot limited to, intake manifold absolute pressure (MAP), cam phaseangle, spark timing, exhaust gas recirculation level, and engine speed.The power train parameter module 206 may direct the firing control unit210 to set selected power train parameters appropriately to ensure thatthe actual powertrain output substantially equals the requested output.The firing control unit 210 may also actuate the cylinder firings. TheNVH reduction module 208 may use the output of the torque model module204 to determine an NVH associated with any particular firing sequenceand set of power train parameters. In certain cases the NVH reductionmodule 208 may direct additional power source/sink 140 to add orsubtract torque from the powertrain 142. It should be appreciated thatthe various modules depicted in FIG. 2 may be combined or configured ina different manner without impacting the overall functionality of thevehicle control system 100.

Torque Profile

In order to determine whether it is necessary to supply a smoothingtorque, and what that smoothing torque should be, it is advantageous toestimate the overall torque profile of the internal combustion engine.This estimate must be done in an accurate, computationally efficientmanner so that the engine torque profile can be predicted in real time.The predicted torque profile may then be used to determine what, if any,smoothing torque is required.

In various approaches, the above smoothing torque may be appliedselectively. That is, many firing fractions and firing sequences deliveran engine torque profile with acceptable levels of NVH, and thus thesmoothing torque need not be applied in those circumstances. In othercircumstances, a firing fraction or firing sequence may generateundesirable levels of NVH. In these cases a smoothing torque may beapplied to reduce NVH to an acceptable level. In other cases a differentfiring fraction or firing sequence may be used that has acceptable NVHcharacteristics. A smoothing torque may optionally be used with thisfiring fraction or sequence. In various embodiments, the smoothingtorque system is arranged to analyze the energy costs of the availableoptions and select the most fuel efficient approach that also brings NVHto acceptable levels.

A single cylinder, normalized torque profile can be used to model theoverall torque profile of a skip fire controlled internal combustionengine. Normalized profiles for fired and skipped cylinders may berecorded in a look up table. Tables can be generated for various levelsof intake manifold absolute pressure (MAP), such as MAP increments of 10kPa. Intermediate values may be determined by interpolation from thesetables. An estimated torque profile for each cylinder can then bedetermined based on scaling and shifting the normalized torque byfactors such as spark and cam phase angle, which controls the openingand closing times of the intake and/or exhaust valves. Differentnormalized profiles can be used for fired and skipped cylinders. Whendifferent firing levels are used, the different firing levels can bemodeled differently by beginning with different normalized profiles foreach different firing level and/or by scaling and shifting differentlybased on different spark and cam settings used. The estimated torqueprofile for all engine cylinders can be summed with the appropriatephasing to obtain the overall engine torque profile. The methoddescribed herein can be used to determine the engine torque with aresolution of 0.5° of crank angle, although as described below, courserresolution can often be used to reduce computational time withoutsignificantly impacting model accuracy.

FIGS. 3A and 3B shows a torque profile associated with two different MAPvalues for an engine operating over a range of speeds. FIG. 3A is for anaverage MAP of 70 kPa and FIG. 3B is for an average MAP of 40 kPa. Inboth cases the vertical scale is torque and the horizontal scale isengine crank angle. Both graphs are for a fired cylinder. The figuresshow the torque profile in increments of 0.5° of crank angle. Thevarious individual cycle profiles shown represent a range of enginespeeds and cam angles. Spark timing has been adjusted for optimum fuelefficiency in all cases.

FIGS. 3A and 3B depict the cylinder torque profile for a 4-strokeengine. Such an engine completes an engine cycle in 720° of crankrotation. An engine cycle can be divided into four phases or strokes,intake, compression, combustion (power), and exhaust. Each strokeextends over 180° of crank angle rotation. The stroke transitionscorrespond to successive top dead center (TDC) and bottom dead center(BDC) piston positions. The torque here is zero, since the lever arm onthe crankshaft is zero at TDC and BDC.

Inspection of FIGS. 3A and 3B shows that the maximum torque generated inthe combustion stroke is significantly higher at 70 kPa compared to 30kPa, since more air and fuel are inducted into the cylinder at higherMAP valves. Also, the pumping losses, denoted by the negative torqueregions in the intake stroke are larger at the smaller MAP value. Skipfire engine operation tends to operate at higher MAP values to minimizethese pumping losses and thereby improve fuel economy.

The torque profiles at each MAP and cam angle may be normalized. FIGS.4A and 4B show such a normalized torque profile for the combustionstroke of an engine cycle with a 30° cam angle and for an average MAP of70 kPa and 40 kPa, respectively. In these figures the vertical axis isnormalized torque and the horizontal axis is crank angle. An important,unexpected observation is that by normalizing the torque profiles to thehighest instantaneous torque, all the normalized torque profilesassociated with each firing are substantially identical for all enginespeeds. FIGS. 5A and 5B show such a normalized torque profile for thecompression stroke of an engine cycle for a 30° cam angle and an averageMAP of 70 kPa and 40 kPa. In this figure the vertical axis is normalizedtorque and the horizontal axis is crank angle. Again all the individualtorque profiles have substantially identical normalized torque profiles.Similar normalized profiles can be generated for the intake and exhauststrokes of a fired cylinder.

Likewise, similar profiles can be generated for a skipped cylinder. Askipped cylinder has no power producing combustion or high temperatureexhaust gases. As such, the “intake” and “combustion” stroke may havegenerally similar profiles when low pressure gas springs are used, asare the “compression” and “exhaust” strokes. The nature of the torqueprofile during a skipped firing opportunity will vary depending on valvemotion during the skipped opportunity. A skipped cylinder may bedeactivated, where either one or both the intake and exhaust valves stayclosed during an engine cycle, so that no air is pumped through thecylinder. If both valves are closed during the cycle, the hot exhaustgases may be trapped in the cylinder or the hot exhaust gases may bereleased prior to closing the valves. These situations may be referredto as forming a “low pressure” spring (venting exhaust gases prior tocylinder deactivation) or a “high pressure” spring (trapping exhaustgases by deactivating the exhaust valve prior to the exhaust of a priorfiring). These cases will have different torque profiles that can bemodeled. In some cases, a skipped cylinder may not deactivate the valvesand may pump air through the cylinder. Again this case can be modeled aswell. To aid in understanding the current invention the following graphsand description will assume that a skipped cylinder is operating in a“low pressure” spring mode, but this is not a requirement.

FIG. 6 shows table 400, which illustrates profiles similar to thoseshown in FIGS. 4A, 4B, 5A and 5B in a table format. In table 400 therows correspond to crank angle and the columns correspond to differentMAP values. The columns are normalized in the tables so that theprofiles associated with each MAP value cover the same area, althoughdifferent types of normalizations may be used. A separate table may beconstructed for each engine stroke, i.e. intake, compression, combustion(power), and exhaust of an activated cylinder. Likewise a separate tablemay be constructed for the two different crankshaft rotations of askipped engine cycle, i.e. the intake/compression rotation and thecompression/exhaust rotation. Separate tables for each stroke orcrankshaft rotation are useful, since depending on the engine operatingconditions the scaling factor may be different between the differentstrokes in any given engine cycle.

Since the normalized torque profile associated with any given firing orskip is known, an estimated torque profile associated with each firingopportunity can be determined by scaling the normalized torque profileby the appropriate scaling factors. FIG. 7 shows a portion of anexemplary table 500 for scaling a normalized torque profile. The tableentries are proportional to the total torque produced in a stroke for agiven MAP (rows in the table) and average engine speed (columns in thetable). The average engine speed in vehicle applications is known on areal time basis based on vehicle sensors that monitor the engine speed.The table shown in FIG. 5 is for a cam phase angle of 30°. Other similartables may be constructed for other cam phase angles. In engines usingdual cams, different tables may use different combinations of intake andexhaust valve timing.

The impact of spark timing on the torque profile may be handled indifferent ways. One method would be to construct tables similar to thatof tables 400 and 500 for different values of spark timing. It is likelythat only a table for the combustion stroke would be necessary, sincespark timing will typically have relatively little impact on the otherengine strokes. An alternative method of handling spark timing would beto generate a spark timing multiplier, which can be multiplied to thevalues in table 500 to adjust for the spark timing. In some embodiments,the impact of varying cam phase angle may be incorporated in the torquemodel by use of a simple multiplier, rather than constructingalternative tables 400 and 500 for different cam phase angles.

An alternative method of including spark timing is to represent actualtorque profiles for various spark timings, i.e. construct a set oftables 400 similar to those shown in FIG. 6, for sets of cam and sparktiming. Then a simple multiplication step between the normalized torqueprofile of table 400 (FIG. 6) and scaling factor of table 500 (FIG. 7)would be all that is required to generate the actual torque profile.

Multiplication of the normalized torque profile of table 400 by theappropriate scaling factor of table 500 provides a real time estimate ofthe torque profile in degrees of crank angle for any given cylinder.Once the estimated torque profile associated with each cylinder has beendetermined, it is a simple matter to simply sum the individual cylindertorque profiles. The cylinder profiles will be offset in crank angle andthus time. For a 4 cylinder, 4 stroke engine the cylinder firings willbe offset by 180° of crank angle. The sum of successive firings andskips associated with all the cylinders is the engine torque profile.FIG. 8 shows an example of such an engine torque profile for a fourcylinder, 4-stroke engine operating at an average engine speed of 1500rpm at a firing fraction of ¾. The vertical axis is total net torquefrom all cylinders and the horizontal axis is crank angle. In thisexample the firing pattern repeats every 720°. There are three enginetorque spikes 813 every 720°, which are associated with the threecylinders that fire per engine cycle. Each of the torque spikes isrelatively short in duration. The skipped firing opportunity shows atorque dip. In this example, cylinder load is approximately 65% of itsmaximum value. Often operating at about 65% of the maximum cylinder loadcorresponds minimizing the brake specific fuel consumption (BSFC). Overan engine cycle the maximum instantaneous delivered torque is more than175 N*m, which may yield unacceptable NVH performance. Without theaddition of a smoothing torque, a less fuel efficient firing fractionmay have to be selected to provide the requested torque.

Scaling Multipliers Based on Firing History

In some embodiments, one or more additional multipliers based on thefiring history may be used to further scale the normalized torqueprofile model to more accurately the delivered torque. These multiplierscan be based on the firing history of the particular cylinder and/or thefiring history of the immediately preceding engine firing opportunities(the firing sequence). During skip fire operation of an engine, theamount of torque provided by any particular firing will vary as afunction of both (a) the firing history of the particular cylinder; and(b) the firing history of the immediately preceding engine firingopportunities. Generally, when other things are equal, a particularcylinder that is fired after it was skipped in its previous workingcycle will generate more torque then when that same cylinder is firedafter it was fired in its previous working cycle. This is due in partdue to differences between the valve actuation schemes between a firedworking cycle that follows a skipped working cycle vs. a fired workingcycle that follows another fired working cycles. More particularly, whena fired working cycle follows another fired working cycle, the exhaustvalve opening from the previous working cycle will typically overlapwith the intake valve opening in the following working cycle. Thiscauses a different amount of air to be introduced to the cylinder ascompared to a circumstance in which the exhaust valve opening does notoverlap with the intake valve opening as typically occurs when a firedworking cycle follows a skipped working cycle in the same cylinder.Another factor that affects the air charge is the cooling of thecylinder which allows more air (and correspondingly, fuel) to beintroduced to the cylinder fired. When the cylinder was skipped in itstwo previous firing opportunities, even more cooling can occur and theair charge (and thus the cylinder torque output) may further increaseaccordingly. With all other parameters being equal, the torque outputfor different firing opportunities of the same cylinder can vary by morethan 10% based on that particular cylinder's firing history. Typically,the skip/fire status of the cylinder's immediately preceding workingcycle has the most significant impact on the torque output of aparticular cylinder during a particular working cycle—however, theeffects can be seen based on the skip/fire status of several previousworking cycles.

Similarly, the overall engine cylinder firing history can also impactthe output of any particular cylinder firing. Generally, when theprevious cylinder in the cylinder firing order was skipped, it does nothave an associated intake event. When no intake event occurs, thepressure within the intake manifold will increase somewhat—which causesmore air to be introduced when an intake event occurs for the followingcylinder in the cylinder firing order. The effects of intake eventsassociated with several preceding cylinders (i.e., the engine firinghistory) affects the air charge somewhat like the individual cylinderfiring history. Again, the torque output for different firingopportunities in an engine cycle can vary by more than 10% based on thethen current engine firing history.

The effects of either or both the cylinder firing history and the enginefiring history can be accounted for by using appropriate firing historybased multipliers taken from firing history tables or other appropriateconstructs.

By way of example, the following two tables illustrate one particulartable implementation that accounts for the effects of the engine firingsequence. The first table illustrates multipliers that are based on thenumber of firings that will have occurred since the last skip. In thisexample, if the present fired cylinder is the first firing in the enginefiring sequence following a skip, a torque multiplier of 1.05 is used.If the present fired cylinder is the second consecutive firing in theengine firing sequence following a skip, a torque multiplier of 1.01 isused. If the present fired cylinder is the third consecutive firing inthe engine firing sequence following a skip, a torque multiplier of 0.98is used. If the present fired cylinder is the fourth consecutive firingin the engine firing sequence following a skip or higher, a torquemultiplier of 0.96 is used. It should be appreciated that this table isparticularly useful when using firing fractions of greater than ½ wherethere is an expectation that the firing sequence generated may includemultiple firings in a row.

Number of Firings After skip Multiplier 1 1.05 2 1.01 3 .98 4 .96

A second table can be used to account for the effects of multiplesequential skips in the firing order immediately before the presentfired cylinder. In this table the number of consecutive skips thatoccurred before the present firing is used as the index. In thisexample, if the present fired cylinder follows a single skip in theengine firing sequence, a multiplier of 0.98 is used. If the presentfired cylinder follows two consecutive skips in the engine firingsequence, a multiplier of 0.99 is used. If the present fired cylinderfollows three consecutive skips in the engine firing sequence, amultiplier of 1.03 is used. If the present fired cylinder follows fouror more consecutive skips in the engine firing sequence, a multiplier of1.04 is used. It should be appreciated that this table is particularlyuseful when using firing fractions of less than ½ where there is anexpectation that the firing sequence generated may include multipleskips in a row.

Number of Skips Before Firing Multiplier 1 0.98 2 0.99 3 1.03 4 1.04

The specific multipliers used in the aforementioned engine firinghistory tables will vary based on a number of engine related factorssuch as the intake manifold dynamics, the nature of the engine, and thecharacteristics of the normalized torque profile.

Separate tables may be used to determine the appropriate multiplier toaccount for the firing history of the cylinder itself. One such tableillustrated below that is suitable for use when the fired cylinder wasskipped in its previous working cycle utilizes the intake manifoldpressure (MAP) and the CAM advance as its indices. In this example, whenthe manifold pressure is 50 kPa, and the cam advance is 0 degrees, amultiplier of 1.0 is used. If the cam advance is 10 degrees, amultiplier of 1.02 is used. If the cam advance is 30 degrees, amultiplier of 1.07 is used. If the cam advance is 60 degrees, amultiplier of 1.10 is used. Suitable values are provided for othermanifold pressures as well. When the current intake manifold pressureand/or the current cam advance is between index values in the table,interpolation can be used to obtain more accurate multipliers.

CAM Advance (degrees) MAP (kPa) 0 10 30 60 30 1 1.02 1.07 1.10 50 1 1.021.07 1.10 70 1 1.02 1.05 1.09 90 1 1.01 1.03 1.04 110 1 1.01 1.02 1.0450

Again, the specific multipliers used will vary based on a variety ofengine related characteristics.

Transformations to the Time Domain

In some implementations, it may be desirable to transform informationavailable in the crank angle domain to the time domain. A rough methodof transforming a crank angle domain to a time domain is to simply usethe average engine speed. We have:

Δt _(avg)=Δ(crankangle)/(average engine speed)  (Eq. 1)

For example if the average engine speed is 1500 rpm, then 0.5° of crankangle equals approximately 0.056 msec, and the crank angle domain can bereadily transformed into a time domain.

Alternatively a more precise method of transforming crank angle intotime may be used. Most vehicles monitor engine speed in real time usingan engine speed sensor. The sensor typically measures the time betweenpassage of successive marks on a flywheel rotating with the engine pasta fixed sensor to determine the engine speed. The mark spacing istypically 6° of crank angle. Variations in the torque supplied to thepowertrain will cause variations in the engine speed, which can bemeasured with the engine speed sensor. For example, the torque spikeassociated with a cylinder firing will cause the engine/vehicle to speedup and a torque dip associated with a skipped firing opportunity willcause the engine/vehicle to slow down.

An engine controller can compare recent variations in engine torque,determined by the previously described torque model, with recentlymeasured variations in engine speed and establish a correlation betweenthe two. The controller may then extrapolate this relationship for thefuture estimated torque profile to help transform a crank angle domaininto a time domain. It should be appreciated that the transformation ofa crank angle domain into a time domain is not limited to the previouslydescribed methods, but any suitable method may be used.

FIG. 9 shows transformation of the torque profile of FIG. 8 into a timedomain rather than a crank angle domain. In this figure the verticalaxis is applied torque and the horizontal axis is time. The variation inthe engine speed with applied torque was included in the transformationto the time base. The total elapsed time in the figure, 240 msec,corresponds to the same three engine cycles depicted in FIG. 8. Asidefrom transforming the horizontal axis from a crank angle domain to atime domain, FIG. 9 also depicts a courser resolution model. In thiscase, the torque profile was modeled in 6° crank increments rather thanthe previously described 0.5° increments. The result is a more stairstep like torque profile. In practice we have found that 6° modelingyields sufficient resolution for engine control and diagnostic purposes.In some cases, even coarser resolution, such as 12°, 30°, or even 60°resolution may be sufficient. An advantage of using courser resolutionis a reduction in memory and computational demands on the engine controlunit. Note that the overall shape of the torque profile is very similarwhether a crank angle domain (FIG. 8) or time domain (FIG. 9) is used,with only slight changes resulting from the transformation.

Application of Torque Profile

Knowledge of the torque profile may be advantageously used in a numberof ways. In particular knowledge of the torque profile associated withupcoming firing opportunities may be used to control a smoothing torqueapplied in parallel to the powertrain to cancel or partially cancelvariations in the overall powertrain torque. This smoothing torque maybe positive (adding torque to the powertrain) or negative (subtractingtorque from the powertrain) or both. The smoothing torque may besupplied by a motor/generator or some other means as previouslydescribed.

An engine controller may determine torque profiles for various firingfractions and firing sequences that deliver the requested torque. Someof these profiles may require application of a smoothing torque toprovide acceptable NVH characteristics. The engine controller may thenselect from this set of firing fractions or firing sequence thatfraction or sequence, which provides the requested torque with a minimumof fuel consumption. Generally the selected firing fraction or sequencewill provide the required torque with each cylinder operating at or nearits optimum efficiency.

A set of torque limit calibration tables may be constructed fordifferent engine speeds and transmission gears. These tables compile themaximum allowed instantaneous torque for different operating conditions.If any point on the torque profile, like that shown in FIG. 9 exceedsthe torque limit value in the calibration table, then that firingfraction or firing sequence is not allowed, unless a smoothing torque isapplied to the vehicle's powertrain. For example, if the calibrationtorque limit 917 corresponding to an engine speed of 1500 rpm and thevehicle being in third gear is 110 N*m, then the torque profile depictedin FIG. 9 would not be allowed, since the maximum instantaneoussignificantly exceeds this value.

In addition to, or in place of, a torque limit calibration table othermeasures of NVH may be compiled. For example, angular jerk, the timederivative of torque, may be determined for different torque profiles.If angular jerk exceeds a certain value within a defined frequencyrange, the firing sequence may be not allowed or a smoothing torque maybe added to reduce angular jerk. In still other embodiments, the limitsmay be expressed in terms of a weighted RMS vibration threshold. Thatis, a weighted RMS average of the instantaneous torque variations may bedetermined and that value may be compared to a maximum permissibleweighted RMS vibration threshold.

FIG. 10 shows a smoothing torque that may be applied to the vehicle'spowertrain by an additional power source/sink to reduce the maximuminstantaneous torque to the calibration limit. In this figure thevertical axis is applied torque and the horizontal axis is time. Apositive applied torque represents torque added to the powertrain and anegative torque represents torque removed from the powertrain.Inspection of FIG. 10 shows there are periods when there is no appliedtorque, periods with a negative applied torque, and periods of positiveapplied torque. The three successive periods of negative torque 1013 inan engine cycle overlap with the portions three torque spikes 813corresponding to the cylinder firings of the internal combustion enginethat exceed the torque limit. The one positive period of applied torqueoverlaps the torque trough associated with the skipped cylinder. Theprofile of the smoothing torque may be chosen to substantially match theshape of the torque profiles associated with a firing cylinder. Thisresults in a more repetitive torque profile, which may be perceived ashaving lower NVH.

It should be appreciated that the portions of the engine torque spikes813 that are offset by the negative torque impulses in the smoothingtorque are quite short in duration, with each impulse corresponding toless than 180 degrees of crankshaft rotation, and typically less than 90degrees of crankshaft rotation.

The amounts of positive and negative power supplied by the additionalpower source/sink can be controlled so that they are equal, less lossesassociated with the energy capture/storage/release system. Control inthis manner will result in the amount of stored energy remainingrelatively fixed about some appropriate level. If more stored energy isdesired the amount of power drawn from the powertrain may be increasedand if less stored energy is desired, the amount of power delivered tothe powertrain may be increased. In some embodiments, the energyextracted from the powertrain is returned (minus losses) within thecyclic pattern, which in some cases is within the same engine cycle.More specifically, the extracted energy is preferably returned within aperiod equal to the degrees of crank angle associated with each firingopportunity (sometimes referred to herein as the firing opportunityperiod) times the denominator of the firing fraction. In an 8 cylinderengine, each firing opportunity is associated with 90 degrees ofcrankshaft rotation (the firing opportunity period); in a 6 cylinderengine, each firing opportunity is associated with 120 degrees ofcrankshaft rotation; and in a 4 cylinder engine, each firing opportunityis associated with 180 degrees of crankshaft rotation. Thus, forexample, when a firing fraction having a denominator of 5 is used (e.g.,⅕, ⅖, ⅗, ⅘) in an eight cylinder engine, the energy is preferablyreturned within 450 degrees of crankshaft rotation (90*5)—whereas a 4cylinder engine operating at the same firing fraction would return itsenergy within 900 degrees of crankshaft rotation (180*5). Of course, theactual period in which the energy will be returned will vary as afunction of both the number of cylinders available and the operationalfiring fraction.

FIG. 11 shows a comparison of the powertrain torque profile between askip fire controlled engine without a smoothing torque and a skip firecontrolled engine practicing the current invention. The dashed linedepicts the torque profile of the internal combustion engine alone,without any compensation. This curve is identical to that shown in FIG.9. The solid line depicts the torque profile of the combination of theengine with a motor/generator that can both add and remove torque fromthe powertrain. It is obtained by adding the smoothing torque of FIG. 10to the internal combustion engine torque profile. Inspection of FIG. 11shows that the instantaneous torque profile always remains below 110N*m, which was the limit in this example. It should be appreciated thatthe torque limit varies with engine speed and transmission gear ratioand may also depend on other variables, such as the tip-in or tip-outrate of the accelerator pedal.

In some embodiments the predicted torque profile may be determined for anumber of future firing opportunities assuming different firingfractions or firing sequences. The prediction may extend at leastseveral firings into the future relative to the current firingopportunity. Preferably the prediction extends far enough into thefuture so that the engine controller can activate/deactivate the enginevalves as appropriate for a fire/skip. This lead time may correspond to3 to 9 future firing opportunities depending on the engine speed andvalve actuation mechanism. In some circumstances both longer and shorterprediction periods may be used. In some embodiments the predicted torqueprofile may extend over the period between making a firing decision andimplementing that firing decision.

The engine controller may determine the NVH and fuel consumptionassociated with several of the firing fractions or firing sequences thatdeliver the requested torque. For some firing fraction or firingsequences a smoothing torque may be required to provide acceptable NVH.The controller may then choose to operate the engine on the firingfraction or firing sequence, and optionally smoothing torque, whichprovides acceptable NVH while minimizing fuel consumption. In making thedecision of the appropriate firing fraction or firing sequence, theengine controller may also consider other variables such as the storagelevel in the energy storage device associated with the auxiliary powersource/sink that provides the smoothing torque as well as the conversionefficiency to and from the energy storage device. The engine controllermay use additional knowledge such as whether the energy in the energystorage device is obtained from the internal combustion engine or someother power source, such as the electric power grid in a plug-in hybrid.Use of this invention will allow operation on previously disallowedfiring fractions improving fuel efficiency.

FIG. 12 schematically illustrates a method 1200 of determining the mostfuel efficient firing sequence according to an embodiment of the currentinvention. In this method one or more candidate firing sequences may begenerated at step 1210 by the firing sequence generator 202 (FIG. 2)based on the torque request. The candidate firing sequences may begenerated by any known method, such as those described in U.S. Pat. Nos.8,099,224, 9,086,020, 9,200,587 and 9,200,575 and U.S. patentapplication Ser. Nos. 14/638,908 and 14/704,630, which are incorporatedherein by reference in their entirety. These sequences are input into atorque model 1220. Also input into the torque model are various engineparameters, such as spark timing, cam phase angle, engine speed, MAP,etc. The torque model 1220 determines the torque profile for thesecandidate firing sequences at step 1230. An assessment may then be madeat step 1240 whether a smoothing torque is necessary for the candidatefiring sequence to provide an acceptable NVH level. The vehicletransmission gear setting may be used in making this assessment. If asmoothing torque is not required, the flow diagram can proceed to step1260. If a smoothing torque is required an assessment is made at step1250 whether there is adequate stored energy to supply the smoothingtorque. If insufficient stored energy is available, that candidatefiring sequence cannot be used. If sufficient energy is available, thenthe method proceeds to step 1260, where the fuel efficiency of theevaluated firing sequences are compared and the firing sequenceproviding optimum fuel efficiency is selected as the operational firingsequence. The method then proceeds to step 1270 where the engine isoperate on the selected operational firing sequence. The method 1200 maybe repeated for each firing opportunity to determine an optimal firingsequence.

Generating a smoothing torque to compensate for internal combustionengine torque variations is an application of the previously describedtorque model. During engine operation variables input to the model mayinclude cam angle (controlling valve timing), MAP, engine speed, sparktiming, crank angle, firing sequence, and firing fraction are known. Thetorque model can generate the instantaneous engine torque profile.Knowing the instantaneous torque at a particular crank angle, an enginecontroller may control the smoothing torque needed to be removed fromthe powertrain, for example, by a generator, or added into thepowertrain, for example, by an electric motor. The electricmotor/generator may be integrated into a single unit in communicationwith an electrical energy storage device, such as a battery orcapacitor.

In the description of FIGS. 9-11 above, the torque profile and thesmoothing torque are shown in the time domain. It should be appreciatedthat in other embodiments the smoothing torque can be determined andapplied in the crank angle domain rather than converting to the timedomain. This can be advantageous in some applications because the crankangle is always available to the engine controller. In such embodiments,the drawing of torque may be directed to starts at “x” degrees and endsat “y” degrees, or the addition of torque may start at “m” degree andends at “n” degrees. As suggested above, the values of “x, y, m, n”might be arranged as a table and determined according to current RPM.

Transient Conditions

The preceding description has generally been directed at selecting theoptimum combination of engine firing fraction, cylinder load, andsmoothing torque during operation under nominally steady-stateconditions. While this is important, a skip-fire controlled vehicle willoften be switching between allowed firing fractions to deliver therequired torque. A historic problem with skip fire, as well as variabledisplacement, engines has been unacceptable NVH generated duringtransitions between the number of firing cylinders i.e. changes in thefiring fraction.

A smoothing torque may be applied during any transition, such as thetransition associated with changing firing fraction levels. As describedin co-pending U.S. patent application Ser. Nos. 13/654,248, 14/857,371and U.S. provisional patent application 62/296,451, which areincorporated herein by reference in their entirety, transitions betweenfiring fraction levels may be the source of unacceptable NVH. Use asmoothing torque during those transitions may shorten the requiredtransition time and reduce the use of fuel wasting spark retard duringthe transition.

One method of handling transient conditions may be referred to asharmonic cancellation. In this method a theoretically predicted enginetorque profile is sent through a specially designed FIR (Finite ImpulseResponse) band-pass filter in the crank angle domain to extract the DSFfrequency components that causes excessive vibration in real time. Theengine torque profile may be determined using the previously describedmethods. The filtered signal can be used to create a smoothing torquevia an electric motor/generator to reduce the overall powertrain torquevariation. The filtering may be accomplished using a set of FIR filtersthat may be run in parallel, each extracting a particular frequency bandin the crank angle domain. An advantage of the harmonic cancellationmethods is that the same filter algorithm can be used for quantifyingDSF caused vibration in both steady state and transient conditions.

Harmonic cancellation provides a real time target torque signal in anumerically efficient way that can be used in hybrid vehicle applicationfor vibration reduction. It may be particularly applicable tomicro-hybrids where the starter motor serves as the motor/generator andenergy storage capacity is limited. This type of system can handle therelatively small and short duration torque requirements associated withfiring fraction transitions, which typically last less than two seconds.

To apply harmonic cancellation, a torque profile may be determined usingthe previously described methods or any other suitable method. Forexample, once a “Fire” or “Skip” decision is made on a cylinder by anECU, a torque waveform is created based on the engine parameters (suchas engine speed, MAP, cam angle, etc.) in the crank domain. The totaltorque waveform may be assembled by combining torque waveforms of allcylinders. The total engine torque signal may then be directed through aset of FIR filters to extract the vibration energy (harmonics) caused byDSF operation. Since lower frequencies tend to have a greater NVHimpact, the filter set may consist of a bandpass filter on the first andsecond DSF orders in the crank angle domain. Filtering in the crankangle domain means that the “frequencies” on the first and second DSForders may be fixed with respect to engine speed, so the filterparameters may not require adjustment with engine speed. The FIR filterscan have a linear phase shift, so that the delays of all filters aresimilar. This minimizes distortion in the filtered signal. The filteredvalues of the engine torque profile may be used to help generate acounter or smoothing torque in the crank angle domain. Phase in and outfunctions, sometimes referred to as a cross fading, may be used whenswitching between filters for smoothing transitions. Alternatively,filtered signal may to directed through a secondary filter to minimizediscontinuities during the transient.

FIG. 13 shows an embodiment of the harmonic cancellation method. Inputsto the method include various engine parameters, such as MAP, cam phaseangle, engine speed, and spark timing. A further input to the model isthe firing fraction or firing sequence, which defines the pattern ofupcoming skips and fires. These values are input into an engine torquemodel as previously described. The engine speed and firing informationmay be input into a filter coefficient determination module. The moduledetermines the filter coefficients for the various DSF orders ofinterest, for example, the first and second order. In some casespreviously used filter coefficients may be used in an upcomingcalculation. The future torque profile and filter coefficients are inputinto a filter bank. The filter bank may be a single FIR filter or mayconsist of an array of FIR filters, one for each frequency band ofinterest. An advantage of using multiple FIR filters is that it allowsapplication of different phase compensation to be applied to offsetdifferent phase shifts in generating a physical torque to thepowertrain. The filter bank is configured to calculate an appropriatesmoothing torque to cancel low order torque oscillations in the crankangle domain. The filter coefficients used in the calculation may besent to the filter coefficient determination module for use in asubsequent calculation.

The output of the filter bank is directed to a crank angle to timedomain conversion module. This module may use the engine speed andcalculated future torque profile to transform the input crank domainsignal to an output time domain signal. The conversion may be simplybased on average engine speed or may optionally include calculated speedvariations based on the calculated torque profile. Output of the timedomain conversion module may be directed to the power electronics unit26 (see FIG. 1) of the motor/generator. The power electronics unit 26controls the motor/generator, which adds or subtracts torque from thepowertrain as specified by the time domain conversion module signal. Theresultant powertrain torque has been smoothed to remove torquefluctuations that would cause undesirable NVH.

FIG. 14 illustrates some of the timing constraints required tosuccessfully practice the methods described in FIG. 13. FIG. 14 shows atime line illustrating decision points, implementation windows, andengine positions associated with some embodiments of implementing themethods described in FIG. 13. At point D a decision is made whether toskip or fire a given cylinder. As described in co-pending U.S. patentapplication Ser. No. 14/812,370, which is incorporated herein byreference in its entirety, that decision is generally made 3 to 9 firingopportunities in advance of the implementation of that decision.Generally it is desirable to minimize the lag between making andimplementation of the firing decision to improve engine responsiveness;however, delays of this magnitude are sufficient for responsive vehiclecontrol. The start of a working cycle corresponding to the firingopportunity associated with the point D decision is denoted as point Son the time line of FIG. 14.

Once the decision to skip or fire is made, the cylinder torque profilefor that firing opportunity can be determined. In FIG. 14 the time tocalculate that torque profile is illustrated as window A. The filterbank has a known delay, which is represented as window B in FIG. 14.This represents the time required for the engine torque signal to beprocessed by the filter bank of FIG. 13. Window C in FIG. 14 representsthe time required for conversion of the filtered signal output by themotor/generator to torque on the powertrain. As long as the endpoint ofwindow C precedes point S, the start of the firing opportunity, theapproach described in conjuction with FIG. 13 may be successfullyimplemented. Window D in FIG. 14 represents the extra, unallocated timeavailable to complete the process if that were to become necessary.

FIG. 15 shows representative filter responses for a variety of firingfraction denominators for a 4 cylinder, 4-stroke engine. Columns in FIG.15 correspond to various firing fractions, n/2, n/3, n/4, and n/5 wheren is an integer greater than zero and less than the denominator and thenumerator and denominator have no common factors. The first row in FIG.15 represents the filter characteristics associated with the first ordervibration of the engine. The horizontal axis on these graphs is anormalized frequency, expressed in terms of engine order. Here an engineorder of one corresponds to one cylinder firing per engine revolution.The second row corresponds to the second order engine vibrationfrequency. The third row corresponds to the composite frequency responseof the two filters. Inspection of FIG. 15 shows that for n/2 the firstorder frequency is at an engine order of 1, i.e. at a firing fraction of½ in 4 cylinder, 4-stroke engine there is one firing per enginerevolution. For the case of n/3 the first order vibration is at anengine order of ⅔, for n/4 the first order vibration is at an engineorder of ½, and for n/5 the first order vibration is at engine order of⅖. The second order frequencies are at twice the frequency of the firstorder. The sum of the two frequency responses are the broader peakedcurves shown in the bottom row. In FIG. 15 the shape of the filtercoefficients has been adjusted to so as to provide a substantiallyconstant, linear phase shift for all filters. While the peak gain isgenerally near a harmonic frequency, peak gain need not correspondexactly with these frequencies. Rather the gain at the harmonicfrequency can be set at a defined value, 1 in the examples shown in FIG.15 and the filter characteristics adjusted to provide for a linear phaseresponse.

FIG. 16 shows an exemplary resultant filtered signal for a specificengine operating condition. In this case the engine is operating withcam phase angle of 40°, a speed of 1500 rpm, a MAP of 50 kPa, and afiring fraction of ⅔. The resultant engine torque profile under theseconditions is shown by curve 1510 in FIG. 16. As expected curve 1510shows two torque spikes, associated with firing cylinders, followed by atorque dip associated with a skipped cylinder. The red curve 1520 andpurple curve 1530 show the filtered signal for crank angle resolutionsof 1° and 30°, respectively. The curves are substantially identical,with at most a 6% difference in the filtered signal value. The relativeinsensitivity of the filtered signal to the filter resolution indicatesthat accurate results can be obtained even using course resolution. Useof course resolution dramatically decreases the computation timerequired to make the calculations, for example, determining the filteredsignal at a resolution of 1° takes approximately 130 times longer thandetermining the resolution at 30°. This allows the calculations to bemade on a real time basis in an ECU or some other vehicle control modulewith only modest processing power and speed.

FIG. 17 shows the resultant suppression of the first and second ordervibrations in the powertrain. In this figure the horizontal axis isengine order, effectively normalized frequency, and the vertical axis isthe amplitude of the powertrain vibration at that frequency. The greycurve shows the response without the addition of any smoothing torque.Inspection of the figure shows significant vibration at an engine orderof 0.5 and 1. The green curve shows the resultant powertrain vibrationswith the addition of the smoothing torque shown generated by thefiltered signal of FIG. 16. As evident in the figure, the first andsecond order oscillations have been almost completely eliminated.

Transient conditions may be handled using a cross fading technique asshown in FIGS. 18A-D. FIG. 18A is the filtered output of one filterbank, denoted as filter A, and FIG. 18B is the filtered output of asecond filter bank, denoted as filter B. The output of the two filterbanks is summed according to a switching function illustrated in FIG.18C. FIG. 18D is a sum of the filtered outputs of filter A and filter Bweighted by the switching function shown in FIG. 18C. While theswitching function is shown as linear in FIG. 18C, this is not arequirement. Use of cross fading allows the filtered signal totransition seamless during a firing fraction transition.

Some of the advantages of the harmonic cancellation method are that itautomatically handles transient conditions. The filtering can beindependent of engine speed and cylinder load. It is energy efficientsince it only damps certain frequency components, which is especiallyimportant in micro-hybrid application. Phase-in and phase-out methodsallow smoothly switching the filters. Furthermore, the method has a lowcomputation overhead for determining the filtering and gain settings andis numerically efficient, both in terms of computation and memory usage.

Exit from DCCO (Decel Cylinder Cut-Off)

One particular transient condition that can occur in a skip firecontrolled engine is DCCO (decel cylinder cut-off). Operation of adynamic skip fire controlled engine during DCCO has been described inco-pending U.S. patent application Ser. No. 15/009,533, which isincorporated herein by reference in its entirety. Use of DCCO improvesfuel economy, since the cylinders are not being fueled duringdeceleration when no torque is being requested (e.g. when theaccelerator pedal is not depressed). Use of DCCO further improves fueleconomy relative to the more commonly used (DFCO) (decel fuel cut-off)because the cylinders during DCCO have been deactivated so that they donot pump air. The pumped air compromises the oxidation/reduction balancerequired in a 3-way catalytic converter, so its use may be limitedand/or extra fuel may be required to restore the catalyst balance.

One problem with DCCO is that the intake manifold fills with air duringa DCCO event. When torque is again requested the high MAP may result inhigh cylinder loads causing a torque surge leading to unacceptable NVH.Solutions to this problem include reducing engine efficiency byretarding spark timing and/or skipping some cylinders withoutdeactivating the valves to help pump down the intake manifold. Boththese solutions have limitations. Retarding spark reduces fuel economy.Pumping air through the engine oxidizes the catalytic converter, whichmay require additional fuel to restore the oxidation/reduction balance,again reducing fuel economy.

During an exit from a DCCO event MAP will generally drop fromatmospheric or near atmospheric pressure to a value appropriate fordelivering the requested torque for example 70 or 80 kPa. The previousdescribed torque model may be used to determine the engine torque whenexiting a DCCO event. In this case, MAP will be changing over successiveengine cycle. MAP changes can be modeled using methods described inco-pending U.S. patent application Ser. No. 13/794,157, 62/353,218, and62/362,177, which are incorporated herein by reference in theirentirety. Other methods of MAP estimation may be used. As the MAP dropsthe output per fired cylinder will generally decrease in a roughlyproportional manner.

The torque surge may be cancelled or reduce by use of a smoothingtorque. The smoothing torque may be chosen so that the powertrain torquegradually increases from zero, the value during DCCO, towards therequested torque level. Unlike some of the previously described casesthe smoothing torque in this case will not necessarily display a regularcyclic behavior and the smoothing torque will generally be removingtorque from the powertrain during the transient period associated withexiting a DCCO event. The energy associated with the removed torque maybe stored in an energy storage device, such as a capacitor or battery,and used to help power the vehicle at a future time. Application of asmoothing torque from the additional power source/sink during an exitfrom a DCCO event improves fuel efficiency and does not impact thecatalytic converter oxidation/reduction balance.

More generally this same type of control strategy may be used wheneverthere is a firing fraction transition from a low firing fraction to ahigher firing fraction. These transitions have a tendency to create anengine torque surge, which can be mitigated by absorbing some or all ofthe excess torque in an energy storage device. Similarly, transitionsfrom a high firing fraction to a low firing fraction may cause a torquedip in engine output. This dip may be partially or completely filled inusing energy from the energy storage device.

Controlling Accessories to Help Manage Torque

In most of the examples given above, the smoothing torque is applied bya bidirectional energy source/sink such as an electric motor/generatorthat is capable of both adding torque to the powertrain and drawingtorque from the powertrain, with the excess energy being stored in astorage device such as a capacitor or battery. Although electric hybridvehicles are particularly well suited for applying the smoothing torque,similar effects can be obtained in some circumstances in non-hybridvehicles through the active control of certain accessories. For examplemost non-hybrid automotive engines include an alternator. Whengenerating electricity, the alternator puts a load on the engine. Duringnormal driving, the alternator is often configured to generateelectricity to charge the battery. The output of an alternator can becontrolled by controlling the field winding current of the alternator.Thus, in some embodiments, the output of the alternator can be varied toload and unload the powertrain in a manner that effectively applies asmoothing torque to the powertrain.

When more power is needed from the engine, the alternator field currentcan be reduced or removed—which will cause the output of the alternatorto drop thereby reducing the load on the powertrain which makes moretorque available to the drivetrain. When less power is needed from theengine, alternator can be commanded to produce more power which providesa higher load on the engine. Thus, the alternator field current can bemodulated in a manner that varies its load on the powertrain to offsetvibration inducing torque surges. Pulse-width modulated signals aretypically used to drive the alternator field current and can readily becontrolled to produce higher (or lower) alternator output voltages tocharge the battery and momentarily increase (or decrease) the drag loadapplied to the powertrain by the alternator. When the battery charge isalready high and more battery charging is not desirable, devices such asthe rear window heater or the front windshield heater can be turned onto absorb the electrical load. The use of the alternator in this manneris particularly effective at handling torque surges in applications suchas transitioning out of DCCO operation back to skip fire operation of anengine.

Another accessory that can sometimes be used in a similar manner is anair conditioner in operating circumstances where the air conditioner isoperating. Specifically, since, the precise output of an airconditioning unit is generally not critical, its output can be modulatedto provide some of the described torque smoothing functions.

Other Embodiments

The embodiments described above have primarily been described in thecontext of smoothing the torque in conjunction with skip fire operationof an engine. However, it should be appreciated that the describedtechniques are equally applicable in embodiment that utilizemulti-charge level or other types of firing level modulation engineoperation. Furthermore, many of the described techniques can be used toimprove operation during traditional variable displacement operation ofan engine—including during both transitions between differentdisplacements and during steady state operation at a particulardisplacement.

Another application of the torque model described above would be enginecalibration. Engine calibration is much easier using this method. Acalibrated table based on engine speed and firing fraction or firingsequence for each gear would indicate what operating conditions provideacceptable NVH. If the engine torque excursions exceeded the allowedtorque, i.e. the output of the vibration calibration table, a smoothingtorque may be added to bring the overall torque profile withinacceptable levels.

The invention has been described in conjunction with specificembodiments, it will be understood that it is not intended to limit theinvention to the described embodiments. On the contrary, it is intendedto cover alternatives, modifications, and equivalents as may be includedwithin the spirit and scope of the invention as defined by the appendedclaims. The present invention may be practiced without some or all ofthese specific details. In addition, well known features may not havebeen described in detail to avoid unnecessarily obscuring the invention.For example, there are many forms of hybrid engines, parallel hybrids,series hybrids, micro hybrids, mild hybrid, full hybrids depending onthe relative size of the two power sources, the storage capacity of theauxiliary energy source, and the mechanisms used to store the auxiliaryenergy. The invention described herein is applicable to all these typesof hybrid vehicles.

In accordance with the present invention, the components, process steps,and/or data structures may be implemented using various types ofoperating systems, programming languages, computing platforms, computerprograms, and/or computing devices. In addition, those of ordinary skillin the art will recognize that devices such as hardwired devices, fieldprogrammable gate arrays (FPGAs), application specific integratedcircuits (ASICs), or the like, may also be used without departing fromthe scope and spirit of the inventive concepts disclosed herein. Thepresent invention may also be tangibly embodied as a set of computerinstructions stored on a computer readable medium, such as a memorydevice.

1. A control system configured to control the transition of an enginebetween different displacements in a hybrid vehicle having an internalcombustion engine and an additional power source/sink, the controlsystem comprising: an engine control unit configured to direct operationof the engine, including directing the transition of the engine from afirst operational displacement to a second operational displacement thatis different than the first operational displacement; a torque profileestimator configured to determine an engine torque profile associatedwith the transition from the first operational displacement to thesecond operational displacement; and an additional power source/sinkcontroller configured to determine a smoothing torque based at least inpart of the determined engine torque profile and to direct theadditional power source/sink to apply the smoothing torque during thetransition from the first operational firing fraction.